What is the different between [ ] and ( ) sign on AP invoice and AR transaction?

I'm wondering about the ( ) on AP invoice and AR transaction form.
I know the [ ] is the DFF but what is the ( )?
When I click on it, it is the Localization Form.
Does we get it when install the Localization? Or after install application, we already get it?
Is there any other function area to meet it?
I have just found only on AP invoice and AR transaction.

I haven't found such () things in my AR Transaction and AP Invoice forms. Can you confirm whether the forms are customized or not? As you said these [] are Descriptive Flex Fields (DFF's). May be they customized the form because in standard one we didn't see such () things.